It's taken eight years but Richard Wilson has finally found a way to exorcise the ghost of Victor Meldrew: by smiting vampires and fighting bad sorcery with good sorcery. It seems that the man once hamstrung by sitcom's most immortal catchphrase has forged himself a new career – but he's in danger of succumbing to the curse of the typecast once more.

This weekend will see Wilson make his debut in Demons as Father Simeon, the wizened spirit guide to thrusting young boy hero Luke Van Helsing. And it comes just weeks after his last role as Gaius - the wizened spirit guide to thrusting young boy hero Merlin. It's all rather confusing, since he even appears to be wearing the same wig.

It's pleasing, of course, that Wilson has found a new lease of life; a trend that began, like most things in the cult world lately, with Doctor Who (Wilson starred as Doctor Constantine in the Stephen Moffatt two-parter The Empty Child). But such an obvious piece of casting speaks of everything that's wrong with ITV's new punt at the Saturday tea-time slot. Demons comes from the same production team as Merlin, so you can forgive a bit of repertory casting. It's just that every road from this Buffy rip-off leads to the most obvious of places.

Wilson's co-star in Merlin is Anthony Head, who of course earned his genre chops as Rupert Giles in Buffy. Giles was an Englishman sent to the US to teach a teenage American Chosen One how to slay vampires. In Demons, Phillip Glenister plays Rupert Galvin, an American sent to England to teach a teenage English Chosen One how to slay vampires. Except they're not actually called vampires in demons, but "inhumans" (what, like animals?). Even Demons's central conceit – that Bram Stoker borrowed from real life for his classic novel – is ripped straight out of the BBC's Jekyll series. Which was written by, yes, Steven Moffatt.

I'm not here to smite Demons before it's even got started, although the plotting and pacing of episode one were all over the place. Demons might well hit its stride before the end of its run, just as Merlin did. Only five years ago, the highest-concept BBC Saturday night telly got was Vic & Bob's Families At War; not long before that, the biggest risk an ITV drama department would take was to cast Colin Firth instead of Robson Green. In never wanting to go back, we'll tolerate things we maybe shouldn't. For every Torchwood there's a Spooks: Code 9.

The start of Demons fell well short of the standard that would normally be required to build a loyal audience. We all know that cult fans will stick with something for longer than they ought to. It seems, sadly, that Richard Wilson is caught in a similar hex.